The present invention relates generally to a reflective mask for use in lithography, such as extreme ultra-violet lithography, and to a methodology for making the same.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. With regard to semiconductor fabrication, a silicon slice (e.g., a wafer) is coated uniformly with a radiation-sensitive film (e.g., a resist). The coated substrate can be baked to evaporate solvents in the resist composition and to fix the resist coating onto the substrate. An exposing source (e.g., light, x-rays, an electron beam) can then be utilized to illuminate selected areas of the surface of the film through an intervening master template (e.g., a mask or reticle) to affect the transfer of a pattern formed within the template onto the wafer. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image from the intervening master template is projected onto the resist coating, it is indelibly formed therein.
Light projected onto the resist layer during photolithography changes properties (e.g., solubility) of the layer of material such that different portions thereof (e.g., the illuminated or un-illuminated portions, depending upon the type of resist utilized) can be manipulated in subsequent processing steps. For example, regions of a negative resist become insoluble when illuminated by an exposure source such that the application of a solvent to the resist during a subsequent development stage removes only non-illuminated regions of the resist. The pattern formed in the negative resist layer is, thus, the negative of the pattern defined by opaque regions of the template. By contrast, in a positive resist, illuminated regions of the resist become soluble and are removed via application of a solvent during development. Thus, the pattern formed in the positive resist is a positive image of opaque regions on the template.
By way of example, prior art FIGS. 1-6 generally depict the fundamental operation of positive and negative type resists in a photolithography process. A cross-sectional side view of a portion of one or more layers of a wafer 100 whereon semiconductor structures are produced is illustrated in the figures to facilitate the explanation. In FIG. 1, a resist layer 102 is deposited on a thin film 104, such as via spin-coating, for example. The thin film 104 may include, for example, silicon dioxide (SiO2) and overlies a substrate 106 that can comprise silicon, for example. In FIG. 2, the resist layer 102 is selectively exposed to radiation 108 (e.g., ultraviolet (UV) light) via apertures 110 formed within a mask or reticle 112 to generate one or more exposed regions 114 in the resist layer 102.
When the exposed regions 114 are made soluble, a positive image of the mask 112 is produced in the resist layer 102. These features 114 are revealed when a specific solvent or developer is subsequently applied across the wafer 100 as illustrated in FIG. 3.
In this situation, the resist material is therefore referred to as a xe2x80x9cpositive resistxe2x80x9d. Areas 116 of the film 104 underlying the removed regions 114 of the resist layer 102 may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask 112 to the film 104, as illustrated in FIG. 4 (wherein the remaining portions of the resist layer 102 has been stripped away or otherwise removed).
Conversely, when the exposed regions 114 are made insoluble by radiation, a negative image of the mask 112 is produced in the resist layer 102. These features 114 remain when the rest of the resist layer 102 is removed via application of a specific solvent or developer across the wafer 100, as is illustrated in FIG. 5. In this situation the resist material is referred to as a xe2x80x9cnegative resist.xe2x80x9d Revealed areas 118 in the film 104 may then be subjected to further processing (e.g., etching) to thereby transfer into the film 104 the desired features 120 from the mask 112, as illustrated in FIG. 6 (wherein the remaining portions of the resist layer 102 have once again been stripped away or otherwise removed).
Projection lithography is a powerful and important tool for integrated circuit processing. However, as feature sizes continue to decrease, optical systems are approaching their limits due to the wavelengths of the optical radiation utilized. A recognized way of further reducing feature sizes is to lithographically image them with radiation of a shorter wavelength. Extreme ultraviolet (EUV) or xe2x80x9csoftxe2x80x9d x-rays, which have wavelengths within a range of about 30 to 700 Angstroms (i.e., about 3 to 70 nm), can, for example, be considered as an alternative radiation source in photolithography processing in an effort to achieve smaller desired feature sizes.
Prior art FIG. 7 is a schematic diagram illustrating the fundamentals of an exemplary EUV lithography system 700. The prior art system 700 depicted in FIG. 1 is designed to delineate a latent image (not shown) of a desired circuit pattern (e.g., having feature dimensions of 0.13 xcexcm and less) onto a wafer 702, and more particularly onto one or more die on the wafer 702, by illuminating a reflective mask 704 with EUV radiation and having at least a portion of that radiation reflected onto the wafer (e.g. via a system of mirrors). The portion of the radiation reflected onto the wafer 702 corresponds to the desired circuit pattern that is to be transferred onto the wafer 702. It will be appreciated that FIG. 7 is a simplified schematic representation of such a system wherein certain components are not specifically shown.
By way of example, EUV radiation 706 having a wavelength of between about 3 nm to 70 nm, for example, can be generated from a light source 708, such as a synchrotron or a laser plasma source that can include optical filtering elements 710 and a reflective condenser 712. The condenser and filtering elements can collect the EUV radiation and project one or more beams 714 onto the reflective mask 704 through a slit (not shown), for example, having a particular width and length. The reflective mask 704 absorbs some of the EUV radiation 716 and reflects other portions of the EUV radiation 718 corresponding to one or more features or circuit patterns formed on the mask. The reflective system can include, for example, a series of high precision mirrors 720 (e.g., concave and/or convex mirrors) which can cause the radiation to converge and/or diverge in projecting a de-magnified or reduced image of the pattern(s) to be transferred onto the wafer 702, which is coated with a resist material. Typically, the reflective mask 704 and wafer 702 are mounted to stages (not shown) such that a scanner can move the mask 704 and the wafer 702 at respective orientations and speeds relative to one another (e.g., in a step and scan fashion) to effect a desired mask-to-image reduction and to facilitate pattern transfers onto one or more different die on the wafer.
The mask 704 of prior art FIG. 7 is an important component in the EUV lithography system 700. Unlike conventional UV lithography systems which predominately use refractive optics, many EUV lithography systems, such as the system 700 depicted in prior art FIG. 7, utilize reflective optics. The mask 704 is thus a reflective mask that reflects at least some incident EUV radiation to transfer a pattern onto a wafer during a semiconductor fabrication process, as opposed to allowing some of the radiation to pass through selected portions of the mask.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, its purpose is merely to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention pertains to a lithography mask or reticle and method of making the same that enhances the fidelity of pattern transfers by reducing the opportunity for contaminating particles to become wedged between the mask and a chuck upon which the mask may rest during semiconductor processing via, for example, electrostatic chucking, and also by facilitating heat dissipation via thermal conductance to mitigate warping of the mask. One or more thermally conductive pads formed within one or more layers applied to the mask facilitate the thermal conductance, and spaces or apertures formed within the layers between the pads mitigate particle contamination.
According to one or more aspects of the present invention, a method of making a reflective lithography mask includes forming a first layer of thermally conductive material over a backside of a substrate of the reflective mask, wherein one or more features to be transferred onto a wafer are formed within one or more layers formed over a topside of the substrate. A second layer of thermally conductive material is then formed over the first layer of thermally conductive material, and one or more thermally conductive pads are then formed within the second layer of thermally conductive material.
According to one or more other aspects of the present invention, a method of making a reflective lithography mask includes forming one or more layers of a thermally conductive material over a backside of a substrate of the reflective mask, wherein one or more features to be transferred onto a wafer are formed within one or more layers formed over a topside of the substrate, and forming one or more thermally conductive pads within at least one of the layers of thermally conductive material.
In accordance with one or more other aspects of the present invention, a method of making a reflective lithography mask includes forming one or more thermally conductive pads within a backside of a substrate of the reflective mask, wherein one or more features to be transferred onto a wafer are formed within one or more layers formed over a topside of the substrate. The one or more pads facilitate conducting heat away from the mask to mitigate distortion of the mask, and the pads are defined by one or more apertures formed within the backside of the substrate. The apertures mitigate the opportunity for contaminating particles to become lodged between the mask and a flat chuck upon which the mask can rest.
According to one or more other aspects of the present invention, a reflective lithography mask includes one or more layers of a thermally conductive material formed over a backside of a substrate of the reflective mask, wherein one or more features to be transferred onto a wafer are formed within one or more layers formed over a topside of the substrate. The mask also includes one or more thermally conductive pads formed within at least one of the layers of thermally conductive material.
In accordance with yet one or more other aspects of the present invention, a reflective lithography mask includes a substrate, wherein one or more features to be transferred onto a wafer are formed within one or more layers formed over a topside of the substrate. The mask further includes one or more thermally conductive pads formed within a backside of the substrate, wherein the one or more pads facilitate conducting heat away from the mask to mitigate warping of the mask. The pads are defined by one or more apertures formed within the backside of the substrate, and the apertures mitigate the opportunity for contaminating particles to become lodged between the mask and a flat chuck upon which the mask can rest.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which one or more aspects of the present invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the annexed drawings.